Waste heat boilers are commonly used for the recovery of thermal energy from process gas in industrial applications. Waste heat boilers typically include an array of metallic tubes that are secured on each end by comparatively thick plates that are referred to in the art as tubesheets. The tubesheets are surrounded by a shell to form a pressure vessel. Hot process gas is introduced to the waste heat boiler at one end, and flows down the inside diameters of the tubes. Boiler feed water is fed to the other side of the tubes within the pressure vessel. Heat is transferred by conduction through the tube walls from the hot process gas to the boiler feed water, thereby producing pressurized steam for use in other processes.
Since the tubesheet has a greater thickness than that of the boiler tube wall, heat transfer is retarded in this region, and tubesheet temperatures at the inlet end can reach levels where certain modes of corrosion are problematic. Accordingly, it is often necessary to protect the front plate of the waste heat boiler and the inlet ends of the tubes from such high temperatures and/or highly corrosive atmospheres. This is commonly achieved using a combination of ceramic ferrules and castable refractory pieces.
One common ceramic ferrule design is a two-piece design including a ferrule head piece, which protects the front face of the waste heat boiler, and a ferrule stem piece, which protects the tube inlet. During assembly, the ferrule stem is inserted into and passes through the ferrule head, and a sealing gasket of refractory ceramic fiber sleeve fills the space between the head and stem to secure the connection through compression of the fiber. One of these ferrule assemblies is inserted into each tube in the waste heat boiler and secured in place by additional fiber provided around the ferrule stem outside diameter and between the boiler tube inside diameter.
FIG. 1 is a perspective view of such a conventionally known two-piece ceramic ferrule, and FIG. 2 is a cross-sectional view of the two-piece ceramic ferrule shown in FIG. 1. As shown, the two-piece ferrule 10 includes a ferrule stem 2 that is inserted into the ferrule head 1. The ferrule stem 2 is held in place by the compressive forces attributed to the location of the ceramic fiber braided rope 4 and the ceramic fiber gasket 5, along with the heat resistant fiber sleeve 3B, which is provided on a portion of the outer diameter (OD) of the stem 2 that is located within the central bore of the head 1 (see FIG. 2). The ferrule stem 2 also includes a flange 21 at one end thereof, which is seated against an annular rim 122 within the central bore of the head 1 to prevent the stem 2 from passing completely through the head 1. A heat resistant fiber sleeve 3C is provided on a portion of the outer diameter of the ferrule stem 2 that remains extended from the ferrule head 1, and the heat resistant fiber sleeve 3A is provided on a portion of the outer surface of the head 1.
As described above, a piece of compressed high temperature ceramic fiber is used to secure the connection between the ferrule head land the ferrule stem of the two-piece ferrule 10. However, when the ferrules 10 are used in service, the forces present can cause separation between the respective heads 1 and the stems 2, resulting in the failure of that ferrule 10 from its intended use. At the minimum, this requires refractory repair and replacement of the ferrules 10 causing production down time, which is costly not only in terms of manpower and parts, but in terms of process down-time and lost productivity. More likely, however, is the potential for catastrophic failure of the boiler and tube due to the thermal degradation and corrosion that results from the ferrules not being properly in place. This scenario is significantly more costly than mere refractory replacement.
One mode of failure of the above-described two-piece design occurs when the stem 2 disassociates from or “backs out” of the head 1, which can be caused by vibrations in the system or any kind of resultant back pressure, which is common during cleaning operations. Once the stem 2 is unseated from its properly assembled position, it is possible for the process gas to bypass the ferrule 10 by transitioning along the stem 2 outer diameter (OD), contacting the waste heat boiler tubesheet area and causing unacceptable heating and subsequent corrosion.